Enumeration of white blood cells

ABSTRACT

A sample acquiring device for volumetric enumeration of white blood cells in a blood sample that includes a measurement cavity for receiving a blood sample. The measurement cavity has a predetermined fixed thickness. The sample acquiring device also has a reagent, which is arranged in a dried form on a surface defining the measurement cavity. The reagent has a hemolysing agent for lysing red blood cells in the sample and a staining agent for selectively staining white blood cells in the sample. Also, a system with the sample acquiring device and a measurement apparatus. The measurement apparatus has a sample acquiring device holder, a light source, and an imaging system for acquiring a digital image of a magnification of the sample The measurement apparatus also has an image analyzer arranged to analyze the acquired digital image for determining the number of white blood cells in the blood sample.

TECHNICAL FIELD

The present invention relates to a sample acquiring device, a method anda system for volumetric enumeration of white blood cells in a bloodsample.

BACKGROUND OF THE INVENTION

Determining a white blood cell count is often important in connection totreating a patient. This analysis may be needed for diagnosing e.g.leukemia, or infectious or inflammatory diseases or for monitoringtreatments. It is desirable to enable analysis results to be obtained asquickly as possible in order to minimize waiting times for patients andenabling a physician to make a decision of treatment and diagnosisdirectly when making a first examination of the patient. It wouldtherefore be preferable to provide an analysis method which may bequickly performed by the physician or a nurse without the need ofsending a test away to a laboratory.

Today, a white blood cell count is normally obtained by staining a bloodsample and microscopically viewing the sample in a special countingchamber, e.g. a Bürker chamber. The counting chamber is provided with agrid dividing the chamber in well-defined small volumes. The white bloodcell count can then be determined by counting the number of blood cellsper box in the grid. The white blood cell count is obtained manually byan analyst, who needs to be experienced in performing the analysis inorder to be able to perform a reliable analysis.

This analysis is time-consuming. Further, since it is performedmanually, the results of the analysis may vary depending on the personperforming the analysis.

There are a few number of existing automated analysis methods fordetermining a white blood cell count. The white blood cell count may bedetermined by means of the Coulter principle, which is based ondetermining cell size and thereby the cell type by sensing an impedance.A method for counting white blood cells by the Coulter principle isdescribed in U.S. Pat. No. 5,262,302.

Another method for determining a white blood cell count is disclosed inU.S. Pat. No. 5,585,246. Here, a blood sample has to be prepared bybeing mixed with a fluorescent dye and ligand complex which tags thewhite blood cells. The sample is introduced into a capillary and isirradiated by a laser source which scans over the sample in thecapillary. The fluorescence is measured in order to determine the numberof white blood cells.

In WO 99/45384, a sample containing chamber having varying thickness isshown. The varying thickness separates different compounds of blood. Theblood sample is stained with a colorant to differentially highlight atleast three different white blood cell types in the blood sample. Thewhite blood cells may be enumerated by using an optical scanninginstrument to view a portion of the chamber.

There is still a need to speed up and simplify existing automatedmethods for determining a white blood cell count such that analysis maybe provided at point of care.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a simple analysis fordetermining a volumetric enumeration of white blood cells. It is afurther object of the invention to provide a quick analysis without theneed for complicated apparatuses or extensive sample preparations.

These objects are partly or wholly achieved by a sample acquiringdevice, a method and a system according to the independent claims.Preferred embodiments are evident from the dependent claims.

Thus, there is provided a sample acquiring device for volumetricenumeration of white blood cells in a blood sample. The sample acquiringdevice comprises a measurement cavity for receiving a blood sample. Themeasurement cavity has a predetermined fixed thickness. The sampleacquiring device further comprises a reagent, which is arranged in adried form on a surface defining the measurement cavity, said reagentcomprising a hemolysing agent for lysing red blood cells in the bloodsample, and a staining agent for selectively staining white blood cellsin the blood sample.

The sample acquiring device provides a possibility to directly obtain asample of whole blood into the measurement cavity and provide it foranalysis. There is no need for sample preparation. In fact, the bloodsample may be sucked into the measurement cavity directly from a prickedfinger of a patient. Providing the sample acquiring device with areagent enables a reaction within the sample acquiring device whichmakes the sample ready for analysis. The reaction is initiated when theblood sample comes into contact with the reagent. Thus, there is no needfor manually preparing the sample, which makes the analysis especiallysuitable to be performed directly in an examination room while thepatient is waiting.

Whereas many existing methods are able to count different blood cellsand even subgroups of blood cells, the sample acquiring device accordingto the invention is specifically adapted to performing volumetricenumeration of white blood cells. The reagent comprises a hemolysingagent which will lyse the red blood cells in the blood sample. Thisdestroys the possibilities to enumerate the red blood cells in thesample. On the other hand, the lysing of the red blood cells simplifiesthe distinguishing and identification of the white blood cells withinthe blood sample.

The invention also provides a method for volumetric enumeration of whiteblood cells in a blood sample. The method comprises acquiring a bloodsample into a measurement cavity of a sample acquiring device, saidmeasurement cavity holding a reagent comprising a hemolysing agent and astaining agent to react with the sample such that the white blood cellsare stained, irradiating the sample with the stained white blood cells,acquiring a digital image of a magnification of the irradiated sample inthe measurement cavity, wherein white blood cells are distinguished byselective staining of the staining agent, and digitally analysing thedigital image for identifying white blood cells and determining thenumber of white blood cells in the sample.

The invention further provides a system for volumetric enumeration ofwhite blood cells in a blood sample. The system comprises a sampleacquiring device as described above. The system further comprises ameasurement apparatus comprising a sample acquiring device holderarranged to receive the sample acquiring device which holds a bloodsample in the measurement cavity, and a light source arranged toirradiate the blood sample. The measurement apparatus further comprisesan imaging system, comprising a magnifying system and a digital imageacquiring means for acquiring a digital image of a magnification of theirradiated sample in the measurement cavity, wherein white blood cellsare distinguished in the digital image by selective staining of thestaining agent. The measurement apparatus also comprises an imageanalyser arranged to analyse the acquired digital image for identifyingwhite blood cells and determining the number of white blood cells in theblood sample.

The method and system of the invention provide a very simple analysis ofa blood sample for determining a white blood cell count. The analysisdoes not require complicated measurement apparatus or advanced steps tobe performed by an operator. Therefore, it may be performed in directconnection to examination of a patient, without the need for a qualifiedtechnician. The measurement apparatus utilizes the properties of thesample acquiring device for making an analysis on a sample of undilutedwhole blood that has been directly acquired into the measurement cavity.The measurement apparatus is arranged to image a volume of the samplefor making a volumetric enumeration of the white blood cells from theone image.

The blood sample is allowed to be mixed with the reagent in themeasurement cavity. Within a few minutes, the reaction of the bloodsample with the reagent will have hemolysed the red blood cells andstained the white blood cells such that the sample is ready for beingpresented to the optical measurement. The blood sample may be mixed withthe reagent by e.g. dispersion or diffusion of the reagent into theblood sample or by actively vibrating or moving the sample acquiringdevice so that an agitation is caused in the measurement cavity.

The sample acquiring device may comprise a body member having two planarsurfaces to define said measurement cavity. The planar surfaces may bearranged at a predetermined distance from one another to determine asample thickness for an optical measurement. This implies that thesample acquiring device provides a well-defined thickness to the opticalmeasurement, which may be used for accurately determining the whiteblood cell count per volumetric unit of the blood sample. A volume of ananalysed sample will be well-defined by the thickness of the measurementcavity and an area of the sample being imaged. Thus, the well-definedvolume could be used for associating the number of white blood cells tothe volume of the blood sample such that the volumetric white blood cellcount is determined.

The measurement cavity preferably has a uniform thickness of 50-200micrometers, and more preferably 100-150 micrometers. This implies thatthe measurement cavity does not force the blood sample to be smearedinto a monolayer allowing a larger volume of blood to be analysed over asmall cross-sectional area. Thus, a sufficiently large volume of theblood sample in order to give reliable values of the white blood cellcount may be analysed using a relatively small image of the bloodsample. For most samples, the white blood cell count is so low thatthere will be only minor deviations due to white blood cells beingarranged on top of each other.

The digital image may be acquired with a depth of field at leastcorresponding to a third of the thickness of the measurement cavity. Ifthe focus of the imaging system is centered within the thickness of themeasurement cavity, the depth of field will be arranged such that theentire thickness of the measurement cavity may be imaged with asufficient sharpness in order to identify the white blood cells. Thisimplies that a sufficient focus is obtained of the entire samplethickness such that the entire thickness of the measurement cavity maybe simultaneously analysed in the digital image of the sample. Bychoosing not to focus very sharply on a specific part of the sample, asufficient focus is obtained of the entire sample thickness to allowidentifying the number of white blood cells in the sample. The depth offield would more preferably correspond to at least half the thickness ofthe measurement cavity or even at least the entire thickness of themeasurement cavity. This would further facilitate the possibilities ofidentifying the white blood cells in the blood sample.

The staining agent may be arranged to selectively stain the nucleus ofthe white blood cells. This implies that the white blood cells may beidentified as coloured dots and therefore easily be counted in a digitalimage.

The staining agent may be any one in the group of Hematoxylin, Methyleneblue, Methylene green, Toluidine blue, Gentian violet, Sudan analogues,Gallocyanine, and Fuchsin analogues. However, it should be appreciatedthat the staining agent is not limited to this group, but many othersubstances may be contemplated.

The hemolysing agent may be a quaternary ammonium salt, a saponin, abile acid, such as deoxycholate, a digitoxin, a snake venom, aglucopyranoside or a non-ionic detergent of type Triton. However, itshould be appreciated that the hemolysing agent is not limited to thisgroup, but many other substances may be contemplated.

The sample acquiring device may further comprise a sample inletcommunicating the measurement cavity with the exterior of the sampleacquiring device, said inlet being arranged to acquire a blood sample.The sample inlet may be arranged to draw up a blood sample by acapillary force and the measurement cavity may further draw blood fromthe inlet into the cavity. As a result, the blood sample may easily beacquired into the measurement cavity by simply moving the sample inletinto contact with blood. Then, the capillary forces of the sample inletand the measurement cavity will draw up a well-defined amount of bloodinto the measurement cavity. Alternatively, the blood sample may besucked or pushed into the measurement cavity by means of applying anexternal pumping force to the sample acquiring device.

The sample may be irradiated by light of a wavelength corresponding to apeak in absorbance of the staining agent. Consequently, the stainedwhite blood cells which contain an accumulation of staining agent willbe detected by a low transmittance of light.

The irradiating may be performed by means of a laser source. The lasersource may provide light of a well-defined wavelength fitting theabsorbance of the staining agent. Further, the laser source providescollimated light, minimizing disturbances of stray light, such that apoint of low transmittance of light will be sharply distinguished.

The irradiating may alternatively be performed by means of a lightemitting diode. This light source may still provide sufficientirradiating conditions for properly distinguishing white blood cellsfrom other matter in the sample.

The digital image may be acquired using a magnification power of10-200×, more preferably 40-100×. Within these ranges of magnificationpower, the white blood cells are sufficiently magnified in order to bedetected, while the depth of field may be arranged to cover the samplethickness.

The analysing comprises identifying areas of high light absorbance inthe digital image. The analysing may further comprise identifying blackor dark dots in the digital image. Since the staining agents may beaccumulated in the nucleus of the white blood cells, the absorbance ofthe light may have peaks at separate points. These points will formblack dots in the digital image.

The analysing may further comprise electronically magnifying theacquired digital image. While the sample is being magnified foracquiring a magnified digital image of the sample, the acquired digitalimage itself may be electronically magnified for simplifyingdistinguishing between objects that are imaged very closely to eachother in the acquired digital image.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described in further detail by way of exampleunder reference to the accompanying drawings.

FIG. 1 is a schematic view of a sample acquiring device in the form of acuvette according to an embodiment of the invention.

FIG. 2 is a schematic view of a sample acquiring device according toanother embodiment of the invention.

FIG. 3 is a schematic view of a measurement apparatus according to anembodiment of the invention.

FIG. 4 is a flow chart of a method according to an embodiment of theinvention.

FIG. 5 is a digital image of a blood sample to be used for volumetricenumeration of white blood cells.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to FIG. 1, a sample acquiring device in the form of acuvette 10 according to an embodiment of the invention will bedescribed. The cuvette 10 is disposable and is to be thrown away afterhaving been used for analysis. This implies that the cuvette 10 does notrequire complicated handling. The cuvette 10 is preferably formed in aplastic material and may be manufactured by injection-moulding. Thismakes manufacture of the cuvette 10 simple and cheap, whereby the costsof the cuvette 10 may be kept down.

The cuvette 10 comprises a body member 12, which has a base 14, whichmay be touched by an operator without causing any interference inanalysis results. The base 14 may also have projections 16 that may fita cuvette holder in an analysis apparatus. The projections 16 may bearranged such that the cuvette 10 will be correctly positioned in theanalysis apparatus.

The cuvette 10 further comprises a sample inlet 18. The sample inlet 18is defined between opposite walls within the cuvette 10, the walls beingarranged so close to each other that a capillary force may be created inthe sample inlet 18. The sample inlet 18 communicates with the exteriorof the cuvette 10 for allowing blood to be drawn into the cuvette 10.The cuvette 10 further comprises a measurement cavity 20 arrangedbetween opposite walls inside the cuvette 10. The measurement cavity 20is arranged in communication with the sample inlet 18. The wallsdefining the measurement cavity 20 are arranged closer together than thewalls of the sample inlet 18, such that a capillary force may draw bloodfrom the sample inlet 18 into the measurement cavity 20.

The walls of the measurement cavity 20 are arranged at a distance fromeach other of 50-200 micrometers, and more preferably 100-150micrometers. The distance is uniform over the entire measurement cavity20. The thickness of the measurement cavity 20 defines the volume ofblood being examined. Since the analysis result is to be compared to thevolume of the blood sample being examined, the thickness of themeasurement cavity 20 needs to be very accurate, i.e. only very smallvariations in the thickness are allowed within the measurement cavity 20and between measurement cavities 20 of different cuvettes 10. Thethickness allows a relatively large sample volume to be analysed in asmall area of the cavity. The thickness theoretically allows white bloodcells to be arranged on top of each other within the measurement cavity20. However, the amount of white blood cells within blood is so low thatthe probability for this to occur is very low.

A surface of a wall of the measurement cavity 20 is at least partlycoated with a reagent 22. The reagent 22 may be freeze-dried, heat-driedor vacuum-dried and applied to the surface of the measurement cavity 20.When a blood sample is acquired into the measurement cavity 20, theblood will make contact with the dried reagent 22 and initiate areaction between the reagent 22 and the blood.

The reagent 22 is applied by inserting the reagent 22 into themeasurement cavity 20 using a pipette or dispenser. The reagent 22 issolved in water or an organic solvent when inserted into the measurementcavity 20. The solvent with the reagent 22 may fill the measurementcavity 20. Then, drying is performed such that the solvent will beevaporated and the reagent 22 will be attached to the surfaces of themeasurement cavity 20.

The reagent 22 comprises a hemolysing agent and a staining agent. Thehemolysing agent may be a quaternary ammonium salt, a saponin, a bileacid, such as deoxycholate, a digitoxin, a snake venom, aglucopyranoside or a non-ionic detergent of type Triton. The stainingagent may be Hematoxylin, Methylene blue, Methylene green, Toluidineblue, Gentian violet, a Sudan analogue, Gallocyanine, or a Fuchsinanalogue. When a blood sample makes contact with the reagent 22, thehemolysing agent will act to lyse the red blood cells such that the redblood cells are mixed with the blood plasma. Further, the staining agentwill accumulate in the nuclei of the white blood cells. The reagent 22should contain sufficient amounts of staining agent to distinctly stainall the nuclei of the white blood cells. Thus, there will often be asurplus of staining agent, which will be intermixed in the blood plasma.The surplus of staining agent will give a homogenous, low backgroundlevel of staining agent in the blood plasma. The accumulated stainingagent in the white blood cells will be distinguishable over thebackground level of staining agent.

The reagent 22 may also comprise other constituents, which may beactive, i.e. taking part in the chemical reaction with the blood sample,or non-active, i.e. not taking part in the chemical reaction with theblood sample. The active constituents may e.g. be arranged to catalysethe hemolysing or staining action. The non-active constituents may e.g.be arranged to improve attachment of the reagent 22 to the surface of awall of the measurement cavity 20.

Within a few minutes, the blood sample will have reacted with thereagent 22, such that the red blood cells have been lysed and thestaining agent has accumulated in the nuclei of the white blood cells.

Referring to FIG. 2, another embodiment of the sample acquiring devicewill be described. The sample acquiring device 110 comprises a chamber120 forming the measurement cavity. The sample acquiring device 110 hasan inlet 118 into the chamber 120 for transporting blood into thechamber 120. The chamber 120 is connected to a pump (not shown) via asuction tube 121. The pump may apply a suction force in the chamber 120via the suction tube 121 such that blood may be sucked into the chamber120 through the inlet 118. The sample acquiring device 110 may bedisconnected from the pump before measurement is performed. Like themeasurement cavity of the cuvette, the chamber 120 has a well-definedthickness defining the thickness of the sample to be examined. Further,a reagent 122 is applied to walls of the chamber 120 for reacting withthe blood sample.

Referring now to FIG. 3, an apparatus 30 for volumetric enumeration ofwhite blood cells will be described. The apparatus 30 comprises a sampleholder 32 for receiving a cuvette 10 with a blood sample. The sampleholder 32 is arranged to receive the cuvette 10 such that themeasurement cavity 20 of the cuvette 10 is correctly positioned withinthe apparatus 30. The apparatus 30 comprises a light source 34 forilluminating the blood sample within the cuvette 10. The light source 34may be an incandescent lamp, which irradiates light in the entirevisible spectrum. The staining agent which is accumulated in the nucleiof the white blood cells will absorb light of specific wavelengths, suchthat the nuclei of the white blood cells will emerge in a digital imageof the sample. If a colour image is acquired, the white blood cells willemerge as specifically coloured dots. If a black and white image isacquired, the white blood cells will emerge as dark dots against alighter background.

The light source 34 may alternatively be a laser or a light emittingdiode. This may be used for increasing contrast in the image such thatthe white blood cells may be more easily detected. In this case, thelight source 34 is arranged to radiate electromagnetic radiation of awavelength that corresponds to an absorption peak of the staining agent.The wavelength should further be chosen such that the absorption of theblood compounds is relatively low. Further, the cuvette walls should beessentially transparent to the wavelength. For example, where Methyleneblue is used as the staining agent, the light source 34 may be arrangedto irradiate light having a wavelength of 667 nm.

The apparatus 30 further comprises an imaging system 36, which isarranged on an opposite side of the sample holder 32 relative to thelight source 34. Thus, the imaging system 36 is arranged to receiveradiation which has been transmitted through the blood sample. Theimaging system 36 comprises a magnifying system 38 and an imageacquiring means 40. The magnifying system 38 is arranged to provide amagnifying power of 10-200×, more preferably 40-100×. Within theseranges of magnifying power, it is possible to distinguish the whiteblood cells. Further, the depth of field of the magnifying system 38 maystill be arranged to at least correspond to the thickness of themeasurement cavity 20.

The magnifying system 38 comprises an objective lens or lens system 42,which is arranged close to the sample holder 32, and an ocular lens orlens system 44, which is arranged at a distance from the objective lens42. The objective lens 42 provides a first magnification of the sample,which is further magnified by the ocular lens 44. The magnifying system38 may comprise further lenses for accomplishing an appropriatemagnification and imaging of the sample. The magnifying system 38 isarranged such that the sample in the measurement cavity 20 when placedin the sample holder 32 will be focussed onto an image plane of theimage acquiring means 40.

The image acquiring means 40 is arranged to acquire a digital image ofthe sample. The image acquiring means 40 may be any kind of digitalcamera, such as a CCD-camera. The pixel size of the digital camera setsa restriction on the imaging system 36 such that the circle of confusionin the image plane may not exceed the pixel size within the depth offield. The digital camera 40 will acquire a digital image of the samplein the measurement cavity 20, wherein the entire sample thickness issufficiently focussed in the digital image for counting the white bloodcells. The imaging system 36 will define an area of the measurementcavity 20, which will be imaged in the digital image. The area beingimaged together with the thickness of the measurement cavity 20 definesthe volume of the sample being imaged. The imaging system 36 is set upto fit imaging blood samples in cuvettes 10. There is no need to changethe setup of the imaging system 36. Preferably, the imaging system 36 isarranged within a housing such that the setup is not accidentallychanged.

The apparatus 30 further comprises an image analyser 46. The imageanalyser 46 is connected to the digital camera 40 for receiving digitalimages acquired by the digital camera 40. The image analyser 46 isarranged to identify patterns in the digital image that correspond to awhite blood cell for counting the number of white blood cells beingpresent in the digital image. Thus, the image analyser 46 may bearranged to identify dark dots in a lighter background. The imageanalyser 46 may be arranged to first electronically magnify the digitalimage before analysing the digital image. This implies that the imageanalyser 46 may be able to more easily distinguish white blood cellsthat are imaged closely to each other, even though the electronicmagnifying of the digital image will make the digital image somewhatblurred.

The image analyser 46 may calculate the number of white blood cells pervolume of blood by dividing the number of white blood cells beingidentified in the digital image with the volume of the blood sample,which is well-defined as described above. The volumetric white bloodcell count may be presented on a display of the apparatus 30.

The image analyser 46 may be realised as a processing unit, whichcomprises codes for performing the image analysis.

Referring to FIG. 4, a method for volumetric enumeration of white bloodcells will be described. The method comprises acquiring a blood samplein a cuvette, step 102. An undiluted sample of whole blood is acquiredin the cuvette. The sample may be acquired from capillary blood orvenous blood. A sample of capillary blood may be drawn into themeasurement cavity directly from a pricked finger of a patient. Theblood sample makes contact with a reagent in the cuvette initiating areaction. The red blood cells will be lysed and a staining agent isaccumulated in the nuclei of the white blood cells. Within a few minutesfrom acquiring the blood sample, the sample is ready to be analysed. Thecuvette is placed in an analysis apparatus, step 104. An analysis may beinitiated by pushing a button of the analysis apparatus. Alternatively,the analysis is automatically initiated by the apparatus detecting thepresence of the cuvette.

The sample is irradiated, step 106, and a digital image of amagnification of the sample is acquired, step 108. The sample is beingirradiated with electromagnetic radiation of a wavelength correspondingto an absorption peak of the staining agent. This implies that thedigital image will contain black or darker dots in the positions of thewhite blood cell nuclei.

The acquired digital image is transferred to an image analyser, whichperforms image analysis, step 110, in order to count the number of blackdots in the digital image.

In FIG. 5, an example of a digital image is shown to indicate thepossibility to identify white blood cells in a blood sample which ishemolysed and stained. This digital image was obtained of a cuvettehaving a cavity thickness of 140 μm and using 50 times magnification.The light source irradiates white light, indicating that the white bloodcells may be identified even though the irradiation is not specificallyadapted to an absorption peak of the staining agent. The staining agentused was Methylene blue. Distinct black dots appear in FIG. 5 indicatingwhite blood cells. The image shown in FIG. 5 is a black and whiteversion of a colour image. The contrast between the white blood cellsand the background appears clearer in the colour image than in the blackand white image reproduced here. The black dots may easily be counted byan image analyser.

In manual methods of counting white blood cells, approximately 200 cellsare typically counted for determining the white blood cell count of theblood sample. The method and apparatus presented here may for example bearranged to count approximately 2000 cells, which gives betterstatistical certainty of the obtained results. A normal, healthy adulthas a white blood cell count of 4-5×10⁹ cells/liter blood. This impliesthat 2000 cells are found in samples having a volume of 0.4-0.5 μl. Forexample, if an area of 1.5×1.5 mm in the measurement cavity having athickness of 140 μm is imaged, the volume being imaged is 0.315 μl. Apart of the acquired image may be selected for analysis. Thus, theacquired image may first be coarsely analysed such that no anomalies areallowed in the part being used for determining the white blood cellcount. The part of the acquired imaged selected for analysis may beselected having an appropriate size so that a sufficient volume of theblood sample will be analysed.

It should be emphasized that the preferred embodiments described hereinis in no way limiting and that many alternative embodiments are possiblewithin the scope of protection defined by the appended claims.

1. A method for volumetric enumeration of white blood cells in a bloodsample, said method comprising: acquiring a blood sample into ameasurement cavity of a sample acquiring device, said measurement cavityholding a reagent comprising a hemolysing agent and a staining agent toreact with the sample such that the white blood cells are stained,irradiating the sample with the stained white blood cells, acquiring adigital image of a magnification of the irradiated sample in themeasurement cavity, wherein white blood cells are distinguished byselective staining of the staining agent, and digitally analyzing thedigital image for identifying white blood cells and determining thenumber of white blood cells in the samples, wherein said digital imageis acquired with a depth of field at least corresponding to a third ofthe thickness of the measurement cavity.
 2. The method according toclaim 1, wherein the blood sample is mixed with the reagent in themeasurement cavity.
 3. The method according to claim 2, wherein themeasurement cavity has a thickness of 50-200 micrometers.
 4. The methodaccording to claim 3, wherein a volume of an analysed sample iswell-defined by the thickness of the measurement cavity and an area ofthe sample being imaged.
 5. The method according to claim 1, wherein thesample is irradiated by light of a wavelength corresponding to a peak inabsorbance of the staining agent.
 6. The method according to claim 1,wherein said irradiating is performed by means of a laser source.
 7. Themethod according to claim 1, wherein said irradiating is performed bymeans of a light emitting diode.
 8. The method according to claim 1,wherein the digital image is acquired using a magnification power of10-200×.
 9. The method according to claim 1, wherein said analysingcomprises identifying areas of high light absorbance in the digitalimage.
 10. The method according to claim 9, wherein said analysingcomprises identifying black dots in the digital image.
 11. The methodaccording to claim 1, wherein said analysing comprises electronicallymagnifying the acquired digital image.
 12. The method according to claim2, wherein the measurement cavity has a thickness of 100-150micrometers, and said digital image is acquired with a depth of field atleast corresponding to a third of the thickness of the measurementcavity.
 13. The method according to claim 1, wherein the digital imageis acquired using a magnification power of 40-100×.